The field of the invention is temperature measurements for household appliances and, more particularly, non-contact temperature measurement devices using infrared radiation in combination with a distribution system to accurately measure the temperature of an object disposed in the chamber of such an appliance, such as a microwave oven.
Many household appliances benefit from use of a temperature sensor (as used herein, "household appliances" refers to apparatus used for heating or cooking food, storing food, cleaning wash items, drying washed items, and the like). In such appliances, such as a refrigerator, stove, or clothes dryer, it may be desirable to know the temperature of objects disposed in chambers of the appliance (e.g., food stuffs in a refrigerator or oven; clothes material in a dryer). Temperature sensing devices, to be effective in such an appliance, must be robust and cost effective, and adapted to provide reasonably accurate measurements. Temperature measurement of objects has been the focus of particular attention in ovens for heating food items. One example is microwave ovens.
Microwave ovens use a magnetron to produce the beam of microwave energy which has high heating power. The microwave oven heats or cooks a multiplicity of substances; for example, cooking food or heating water, heating consumable and non-consumable products in or devoid of associated packaging, or even drying wet items. As used herein, the term "object" is meant to include all substances and products heated or cooked by the microwave oven. The microwave energy or beam is directed into a chamber of the microwave oven so as to intercept and enter, in many directions, the object therein, heating and cooking the object. It is desired that the temperature of the object being heated, cooked or warmed in a microwave oven be known. Microwave ovens commonly use algorithms based on time or energy levels (usually selectable from 1 to 10) selected by an operator to allow cooking of the object to its desired temperature.
Different approaches have been tried to monitor the state of the object being heated. One technique for physically measuring the temperature is to insert a temperature sensing probe into the object being heated. This approach necessitates locating the probe, cleaning it, placing it in the object being heated, and finally plugging it into the proper connector of the microwave oven. Aside from the inconvenience to the operator, another drawback of this technique is that it cannot be used for objects that are heated in containers. Another technique uses the water vapor being liberated from the object as an indication of the heated object's temperature and completion of the cooking process. Such an approach requires that the object to be heated contain sufficient moisture to be detected by the measurement system, which limits the usefulness of this approach. A further technique, such as that described in U.S. Pat. No. 4,568,201, uses infrared radiation to provide for a non-contact temperature measurement of the object being cooked and employs an infrared (IR) sensor along with a temperature sensor to control microwave oven cooking. Both of these sensors are located in the oven chamber.
It is desired that a non-contact temperature measurement of food being cooked be provided by using infrared radiation, but not be limited to employing both an infrared (IR) sensor and an independent temperature sensor, and also not be limited to having these sensors located in the chamber of the microwave oven.
Further, the use of microwave energy for heating or cooking an object in a microwave oven may result in the creation of standing waves which have maximum and minimum points spatially distributed in the oven interior. This wave distribution can cause uneven heating or uneven temperature measurement of the object exposed to the radiation. One solution for negating the effect of standing waves on cooking includes mode stirring, which cyclically disturbs the standing waves within the chamber of the microwave oven so as to more evenly distribute the microwave energy throughout the chamber and, accordingly, more evenly heat the object in the chamber of the microwave oven.
Additionally, the attenuation of the microwave radiation between the source, the object being heated, and the measurement device is beneficially considered to provide an accurate temperature measurement of the object being heated.
It is thus desirable to provide a non-contact temperature measurement system for an appliance that can provide a temperature signal calibrated for conditions of the appliance (age, level of dirt or other contamination in the chamber or on the detector, and the like). It is also desirable that the temperature measurement system for an appliance, such as a microwave oven, serve both as an infrared (IR) sensor that detects infrared radiation emanating from the object being heated and as its own temperature sensor for calibration purposes. Such a microwave oven system also desirably has the measurement circuit components located outside the chamber of the oven. It is further desired to employ mode stirring techniques in combination with infrared radiation to provide for a non-contact temperature measurement of the objects being heated or cooked in the chamber of the microwave oven. It is still further desirable to provide for an infrared non-contact temperature measurement of an object being heated or cooked in the chamber of the microwave oven that takes into account the emissivity of the object so that an accurate temperature measurement of the object may be obtained.